Re: [PATCH 8/9] rust: list: support heterogeneous lists

[Alice Ryhl]

On Thu, Apr 4, 2024 at 5:35 PM Benno Lossin <benno.lossin@xxxxxxxxx> wrote:
>
> On 02.04.24 14:17, Alice Ryhl wrote:
> > @@ -180,6 +184,41 @@ unsafe fn from_fields(me: *mut ListLinksFields) -> *mut Self {
> >      }
> >  }
> >
> > +/// Similar to [`ListLinks`], but also contains a pointer to the full value.
> > +///
> > +/// This type can be used instead of [`ListLinks`] to support lists with trait objects.
> > +#[repr(C)]
> > +pub struct ListLinksSelfPtr<T: ?Sized, const ID: u64 = 0> {
> > +    /// The `ListLinks` field inside this value.
> > +    ///
> > +    /// This is public so that it can be used with `impl_has_list_links!`.
> > +    pub inner: ListLinks<ID>,
> > +    self_ptr: UnsafeCell<MaybeUninit<*const T>>,
> > +}
> > +
> > +unsafe impl<T: ?Sized + Send, const ID: u64> Send for ListLinksSelfPtr<T, ID> {}
> > +unsafe impl<T: ?Sized + Sync, const ID: u64> Sync for ListLinksSelfPtr<T, ID> {}
>
> Missing SAFETY comments.

Will do.

> > +
> > +impl<T: ?Sized, const ID: u64> ListLinksSelfPtr<T, ID> {
> > +    /// The offset from the [`ListLinks`] to the self pointer field.
> > +    pub const LIST_LINKS_SELF_PTR_OFFSET: usize = core::mem::offset_of!(Self, self_ptr);
> > +
> > +    /// Creates a new initializer for this type.
> > +    pub fn new() -> impl PinInit<Self> {
> > +        // INVARIANT: Pin-init initializers can't be used on an existing `Arc`, so this value will
> > +        // not be constructed in an `Arc` that already has a `ListArc`.
> > +        Self {
> > +            inner: ListLinks {
> > +                inner: Opaque::new(ListLinksFields {
> > +                    prev: ptr::null_mut(),
> > +                    next: ptr::null_mut(),
> > +                }),
> > +            },
>
> Why don't you use `inner <- ListLinks::new(),`?

Because I wasn't using the macro at all. I was just using the fact
that T implements PinInit<T>. But as discussed on another patch, I'll
replace this entire method with init::zeroed().

> > +            self_ptr: UnsafeCell::new(MaybeUninit::zeroed()),
> > +        }
> > +    }
> > +}
> > +
> >  impl<T: ?Sized + ListItem<ID>, const ID: u64> List<T, ID> {
> >      /// Creates a new empty list.
> >      pub const fn new() -> Self {
>
> [...]
>
> > @@ -94,5 +137,45 @@ unsafe fn post_remove(me: *mut ListLinks<$num>) -> *const Self {
> >              }
> >          }
> >      };
> > +
> > +    (
> > +        impl$({$($generics:tt)*})? ListItem<$num:tt> for $t:ty {
> > +            using ListLinksSelfPtr;
> > +        } $($rest:tt)*
> > +    ) => {
> > +        unsafe impl$(<$($generics)*>)? ListItem<$num> for $t {
> > +            unsafe fn prepare_to_insert(me: *const Self) -> *mut ListLinks<$num> {
> > +                let links_field = unsafe { Self::view_links(me) };
> > +
> > +                let spoff = ListLinksSelfPtr::<Self, $num>::LIST_LINKS_SELF_PTR_OFFSET;
> > +                let self_ptr = unsafe { (links_field as *const u8).add(spoff)
> > +                    as *const ::core::cell::UnsafeCell<*const Self> };
> > +                let cell_inner = ::core::cell::UnsafeCell::raw_get(self_ptr);
> > +
> > +                unsafe { ::core::ptr::write(cell_inner, me) };
> > +                links_field
> > +            }
> > +
> > +            unsafe fn view_links(me: *const Self) -> *mut ListLinks<$num> {
> > +                unsafe {
> > +                    <Self as HasListLinks<$num>>::raw_get_list_links(me.cast_mut())
> > +                }
> > +            }
> > +
> > +            unsafe fn view_value(links_field: *mut ListLinks<$num>) -> *const Self {
> > +                let spoff = ListLinksSelfPtr::<Self, $num>::LIST_LINKS_SELF_PTR_OFFSET;
> > +                let self_ptr = unsafe { (links_field as *const u8).add(spoff)
> > +                    as *const ::core::cell::UnsafeCell<*const Self> };
> > +                let cell_inner = ::core::cell::UnsafeCell::raw_get(self_ptr);
> > +                unsafe {
> > +                    ::core::ptr::read(cell_inner)
> > +                }
> > +            }
> > +
> > +            unsafe fn post_remove(me: *mut ListLinks<$num>) -> *const Self {
> > +                unsafe { Self::view_value(me) }
> > +            }
> > +        }
> > +    };
>
> The paths in this macro should use `$crate::...` to prevent import
> errors.

Will do.

Alice